1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device and, more particularly, to a pixel circuit of an OLED display device.
2. Description of the Related Art
Among flat panel displays (FPDs), an organic light emitting diode (OLED) display device is quite appropriate for displaying moving images irrespective of size because it has a fast response time of 1 ms or less, consumes low power, and is an self-emissive display, therefore having a wide viewing angle. Also, the OLED display device can be fabricated at a low temperature and in a simple process based on conventional semiconductor manufacturing technology. For these reasons, the OLED display device has attracted considerable attention as an advanced FPD.
In general, the OLED display device is a display device that emits light by electrically exciting a fluorescent organic compound, and displays an image by voltage programming or current programming N×M organic light emitting diodes (OLEDs) arranged in a matrix. The OLED display device may be classified as a passive matrix type or an active matrix type using a thin film transistor (TFT) depending on a driving method. The passive matrix type OLED display device includes anodes and cathodes arranged at right angles, and is driven through line selection. On the other hand, the active matrix type OLED display device has a TFT connected to each pixel electrode, formed of indium tin oxide (ITO), and is driven according to a voltage maintained by a capacitor connected to a gate of the TFT.
FIG. 1 is a circuit diagram of a pixel circuit of a conventional OLED display device.
Referring to FIG. 1, the pixel circuit includes a switching transistor MS, a capacitor Cgs for storing a data signal received through the switching transistor MS, a driving transistor MD for generating a driving current in response to the data signal stored in the capacitor Cgs, and an OLED that emits light in response to the driving current.
The switching transistor MS transmits a data signal output from a data line Dm in response to a scan signal output from a scan line Sn. The capacitor Cgs stores a data signal received through the switching transistor MS and maintains a gate-source voltage Vgs of the driving transistor MD for a predetermined duration of time. A gate electrode of the driving transistor MD is connected to the switching transistor MS. The driving transistor MD supplies a driving current corresponding to the data signal transmitted through the switching transistor MS to the OLED. The OLED includes an anode, a cathode, and an emission layer interposed between the anode and the cathode. The anode is connected to a drain terminal of the driving transistor MD and coupled to a power supply voltage line Vdd through the driving transistor MD. The cathode is connected to a reference voltage line Vss and emits light with a brightness corresponding to the current supplied from the driving transistor MD.
In the OLED, holes transported from the anode combine with electrons transported from the cathode in an organic layer to form hole-electron pairs, i.e., excitons, and light is emitted by the energy generated when the excitons transition from an excited state to a ground state.
When the OLED display device including the above-described OLED begins a display operation, a high voltage is applied to a display panel in a short period of time. Thus, a large current is supplied to the OLED suddenly. As a result, sudden transport of a large number of holes and electrons electrically shocks an organic layer of the OLED, so that the characteristics of the OLED may deteriorate.